Laser amplifier

ABSTRACT

A laser amplifier in which the effective refractive index of the active region for the TE and TM modes is sufficiently different to provide multiple mode ripple beat frequency overlap within a 3dB gain envelope peak. The active region is made highly asymmetric with an aspect ratio of at least 10:1 and preferably 20:1 in order to provide the effective refractive index difference. Steps, grading or uniform difference in the material refractive index may also be used. The confinement factor ratio is made as high as possible by providing different material refractive index in the orthogonal dimensions of the active region cross section.

FIELD OF THE INVENTION

This invention relates to laser amplifiers.

BACKGROUND AND SUMMARY OF THE INVENTION

In semiconductor laser amplifiers it is generally found that it isdifficult to have identical confinement factors for the orthogonalpolarisations of input light, which propagate within the active regionor, more generally, within the waveguide region (of which the activeregion forms the core and the cladding includes additional layers, asthe TE and TM modes. Usually there is a higher peak gain for the TE modethan for the TM mode. The result of this is that for laser amplifiersthere is a variation in the gain of the amplifier dependent upon thepolarisation of the input optical signal. In practical terms thispolarisation sensitivity means that the amplifier can not be operatedconsistently at maximum gain and as the polarisation changes theamplifier bias has to be adjusted to increase or decrease the gain to aconsistent operating level.

One solution to polarisation sensitivity is to utilise a polarisationscrambler on the light input to the laser amplifier, but this has thedisadvantages of a reduction in gain (usually 3dB) and, moresignificantly, at high bit rates a degraded signal to noise ratio due tonoise originating in the scrambler.

Within a waveguide the effective refractive index is a function of therefractive index of the material and also the dimensional ratio of thewaveguide, so that an asymmetric waveguide or active region has anasymmetric effective refractive index. This asymmetric refractive indexarises from a difference in the propagation constants for TE and TM. Thedifference in propagation constants may also lead to a difference in TEand TM confinement factors which will in turn result in different TE andTM gains. One way around the problem of different TE and TM gains wouldbe to fabricate symmetrical waveguides, but in practice such a structureis difficult to fabricate and does not lend itself to efficient opticalcoupling to and from the amplifier. It has however been the trend, asfar as possible, to fabricate waveguide regions with minimum asymmetrybecause this produces a more symmetrical spot and eliminates the needfor anamorphic coupling lenses.

An alternative approach would be for a laser to be fabricated that hasan asymmetric active region but nevertheless has similar confinementfactors for TE and TM. Devices have been fabricated with confinementfactor ratios as close as 0.9 (within this specification confinementfactor ratios are given as TM/TE, and are therefore generally less thanunity) in which the gain peaks for TE and TM have a gain difference ofonly 2.5dB (apparently enabling operation within the top 2.5dB gain) andthus this appears to offer an improvement over the 3dB loss andattendant noise of a polarisation scrambler. However, within thewaveguide region of such devices although the confinement factors havebeen closely similar giving close gain envelope peaks, there arelongitudinal mode ripples superimposed on the gain characteristic whichresult from residual cavity reflectivity. Due to the asymmetry ofeffective refractive index resulting from waveguide asymmetry thelongitudinal mode ripples are of a slightly different frequency for thedifferent polarisations and are not in general in phase so that althoughthe peaks of the 3dB ripple envelopes of TE and TM gain may lie only2.5dB apart the actual gain difference at a given wavelength is usuallygreater than this, for example about 5dB, because of the longitudinalmode ripples being out of phase.

The approach adopted in the present invention is to make the effectiverefractive index in the waveguide region for the different polarisationssufficiently different to provide frequent longitudinal mode ripplecoincidence within the gain envelope peak.

Accordingly the present invention provides a laser amplifier in whichthe active region has an effective refractive index difference for lightpropagating in the TE and TM modes such that one of the TE and TM modeshas a greater number of longitudinal modes oscillating within the 3dBgain envelope than the other, and such that at least four longitudinalmodes oscillating in the TE mode substantially coincide in wavelengthwith longitudinal modes oscillating in the TM mode within said 3dB gainenvelope.

According to another aspect of the invention there is a laser amplifierhaving a cavity in which there is an effective optical path lengthdifference between TE and TM propagating polarisations of at least2.5×10⁻⁴ meters.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is now described by way of example with reference to theaccompanying drawings in which:

FIG. 1 is a schematic plot of gain curves for a travelling wave laseramplifier;

FIG. 2 is a schematic plot similar to FIG. 1 for a laser having agreater mismatch in refractive indices for TE and TM propagation; FIG. 3shows gain envelopes for a tested laser;

FIG. 4 shows the gain envelope for another tested laser, and

FIG. 5 is a cross section through part of an embodiment of theinvention.

DETAILED DESCRIPTION OF THE DRAWINGS

In semiconductor laser amplifiers residual facet reflectivity gives riseto longitudinal mode ripples superimposed on the gain characteristic.Anti-reflection coatings applied to the facets may be used to minimisethe ripple: at present facet reflectivities of 10⁻⁵ have been achievedwhich reduces ripple to 1dB for 20 dB gain, but more commonly facetreflectivities of 10⁻³ and ripple of 3dB has to be tolerated. The moderipples in laser gain characteristics depend for frequency on theeffective path length of the active region, i.e. on the effectiverefractive index and cavity length, and upon facet reflectivity foramplitude.

Referring to FIG. 1 a schematic gain versus wavelength plot is shown fora laser amplifier in which the full line represents light propagating inthe TE mode, excited by one input polarisation and the dotted linerepresents light propagating in the orthogonal TM mode excited by theorthogonal input polarisation. The illustrated plot shows the usualeffect of the longitudinal mode peaks for the orthogonal propagationmodes being misaligned due to their occurrence at slightly differentwavelengths and with slightly different spacing. Due to a relativelysmall difference in the effective path lengths for the orthogonalpropagation modes the number of instances of longitudinal mode peakmatching (or beat frequency) is small, and so the probability ofmatching occurring at the gain peak is very small. It is for this reasonthat at particular operating wavelengths the gain sensitivity isgenerally found to be greater than that expected from the gaincharacteristic usually considered, which is the gain envelope.

FIG. 2 shows a similar schematic plot to that of FIG. 1, however in thisinstance the plot is drawn for a device in accordance with the presentinvention that is fabricated with a difference in effective refractiveindex for the orthogonal propagation modes such that there is a greaterdifference in the longitudinal mode frequency. In this instance matchingof the longitudinal mode peaks occurs more frequently and thus theprobability of matching occurring within the gain peak is muchincreased. (In both FIGS. 1 and 2 the TE and TM gain envelopes are shownat substantially equal levels: this degree of equalisation would in factbe difficult to achieve, the figures are drawn to demonstrate the modecoincidence aspect).

The present invention proposes fabrication of a device with differencesin the effective refractive index such that one of the TE and TM modeshas a greater number of longitudinal modes oscillating within the 3dBgain envelope than the other, and such that at least four mode peaks ofthe TE ripples are in phase with mode peaks of the TM ripples. This willprovide a greater probability of longitudinal mode peak alignment at thegain peaks, and/or also a greater number of wavelengths at which thegains are matched. The confinement factors should be made as close aspossible so that the ripple envelopes are as close as possible.

In order to investigate the relationship between confinement factors andeffective refractive index various existing 500 micron cavity lengthdevices were investigated for mode peak coincidence and the results areshown in Table 1.

                  TABLE 1                                                         ______________________________________                                             Ave-   Difference                                                                              TM/TE    Gain    Actual                                 De-  rage   in        Confinement                                                                            Envelope                                                                              peak                                   vice R.I.   R.I.      Factor Ratio                                                                           Difference                                                                            difference                             ______________________________________                                        A    3.5    0.03      0.91     2.5 dB    5 dB                                 B    3.5    0.08      0.91     2.5 dB  2.5 dB                                 C    3.5    0.08      0.85     3.0 dB  3.0 dB                                 D    3.5    0.03      0.76     4.5 dB  6.0 dB                                 E    3.5    0.08      0.76     4.5 dB  4.5 dB                                 ______________________________________                                    

It will be observed from the table that the refractive index differencecan differ for a similar confinement factor. Devices B, C and E wereexceptional in that they had unusually large differences in refractiveindex for TE and TM. The longitudinal mode envelopes for two of thedevices, D and E are shown in FIGS. 3 and 4. The gain differences inthese instances between the longitudinal mode envelopes are greater dueto the lower confinement factor ratio, but nevertheless they demonstratethe frequency with which actual gain difference at peak gainapproximates to the longitudinal mode envelope gain difference.

For device D, over the usable bandwidth of the device, which is usuallydefined as the range over which there is a maximum change of 3dB in thepeak gain of the longitudinal mode envelope, there was only one in phaselongitudinal mode peak at a wavelength of approximately 1.49 microns anda second, just beyond the 3dB bandwidth at a wavelength of approximately1.52 microns, these points are marked with arrows A, while an out ofphase point is marked, at the peak gain, by arrow B. This device istypical of the majority of available laser amplifiers and, although itmay be possible to lower the gain envelope gain difference by increasingthe confinement factor ratio (e.g. to the ratio for device B), thefrequency with which the actual gain difference equals the gain envelopegain difference is very small and, as its location is random, it is notnecessarily at the desired wavelength of the incoming signal or close tothe gain peak of the gain envelopes.

FIG. 4 shows similar plots for one of the devices that was selectedbecause of its unusually high difference in refractive index, in thiscase 0.08, in an average of 3.5. In this instance two in phaselongitudinal mode peaks were located in the 3 dB bandwidth atwavelengths of approximately 1.49 microns and 1.51 microns, thusdemonstrating an increase in frequency of occurrence over the laseramplifier of FIG. 3, but for practical purposes (and even if the gainenvelope gain difference were lower) this is an insufficient number ofwavelengths at which matching occurs for practical purposes.

In accordance with the invention it is proposed that devices arefabricated so that there are at least four wavelengths within the 3 dBlongitudinal mode envelope bandwidth at which TE and TM propagationmodes have coincident longitudinal modes. For a 500 micron cavity lengththis can be achieved with an effective refractive index difference of0.16: for a 1000 micron device it could be reduced to a difference of0.08.

In numerical terms this may be expressed in terms of the optical pathlength difference which is the optical path length times difference ineffective refractive index. ##EQU1## where L is the cavity length of thelaser n is the effective refractive index difference, and n* is theeffective refractive index and is defined by

    n*=c/2LδU

where c is the velocity of light and δU is the mode spacing of thelaser. Typical values of n* are 3.49 for TM and 3.61 for TE.

Preferably the ratio of the TM/TE confinement factors is at least 0.9,and the longitudinal mode occurrence frequency such that coincidence inwavelength of longitudinal modes propagating in TE and longitudinalmodes propagating in TM occur at least once every ten longitudinalmodes.

The effective refractive index n* is different for TE and TM propagationmodes because it is also influenced by the aspect ratio of the waveguidefollowing the general formula: ##EQU2## δβ is the difference inpropagation constants for TE and TM a is the height of the waveguide

b is the width of the waveguide

2Δ=(1-n₁ ² /n₂ ²) where n₁ is the refractive index in the waveguide andn is the refractive index in the cladding ##EQU3## where n is therefractive index of the layer of propagation, in this instance that ofthe waveguide (n₁).

It is this difference in n* that gives rise to the different ripplefrequencies. For a 500 micron cavity length the aspect ratio (wherepropagation is along the length) required is preferably in excess of10:1, more preferably in excess of 15:1 and most preferably in excess of20:1. Having this degree of asymmetry in the waveguide makes the naturalconfinement across the height or shorter dimension much less than acrossthe width or wider dimension. In order to equalise the confinementfactors, as far as possible, the waveguide is bounded by layers whichwill increase the relative confinement across the short dimension. Theboundary layers, or confinement layers, may be made by stepping orgrading the refractive index of the material around the waveguide, forexample by different doping of the material or by different materialcomposition, with the step in refractive index at the top and bottom ofthe waveguide being different (larger) than the step at the sides, theratio of the material refractive index changes being in generalcorrespondent with the aspect ratio.

It is also possible to utilise birefringent material, either in theboundary layers or in the waveguide itself. Within the waveguidebirefringent material may be utilised to provide or assist in providingthe necessary degree of refractive index difference without requiring,or enabling a reduction in the required, waveguide asymmetry.

A preferred embodiment of the invention is shown in FIG. 5, whichillustrates a cross section through the active (or waveguiding)confinement layers of a laser amplifier. The device has an active layer1 that is highly asymmetric having a width of the order of 3 microns anda depth of 0.15 microns. This strong asymmetry results in differentpropagation constants and thus in different effective refractive indexfor the TE and TM modes. In order to match the amplitude of the TE andTM gain envelopes as closely as possible the confinement factors need tobe equalised as closely as possible, and this is achieved by having adifferent material refractive index step between the layer 1 andlaterally bounding regions 2 than between the layer 1 and layers 3 aboveand below the active region. The absolute value of the steps must besuch as to enable maintenance of single mode propagation within thewaveguide. For example the required steps may be +0.02 (in 3.5) aboveand below the waveguide and of the order of +0.001 laterally of thewaveguide.

For devices of differing aspect ratio the approximation waveguidedimension X index step=constant will generally apply.

The precise doping profiles necessary for a device of given dimensionsand the optimum absolute value of the material refractive indices toachieve the desired confinement may be designed iteratively using anequivalent index method programme which will predict the confinementfactors, propagation constants, gain ripple and gain envelope.

The materials from which the device may be fabricated are any of thosefrom which a junction diode can be fabricated, including galliumarsenide, gallium phosphide, indium arsenide, indium phosphide andtertiary and quaternary materials formed from these.

In summary, the present invention therefore proposes overcoming thepolarisation sensitivity problem in laser amplifiers by increasing thedifference in effective refractive index for TE and TM propagation modesto the level at which the frequency of overlap of the longitudinal moderipples of TE and TM propagating light within the 3 dB mode envelopepeak is significant In this way if the confinement factors are also madeequal, or as close as possible, at the wavelengths corresponding to thelongitudinal mode ripple peak overlaps the device is substantiallypolarisation insensitive. A practical way of bringing about thedifference in effective refractive index is to have a highly asymmetricwaveguide and to provide asymmetric lateral and vertical confinementsteps or grading in the material of the refractive index. The problemsthat arise as a result of such a structure are that the spot isasymmetric and the device is wavelength sensitive. However, as far asspot asymmetry is concerned, that can be overcome, for example bycoupling through a cylindrical lens and is acceptable trade off forgetting polarisation insensitivity. (Suitable birefringent material mayalso remove this problem by replacing waveguide asymmetry, for exampleusing a quantum well or multiple quantum well structure).

With respect to the wavelength sensitivity again this is acceptablewhere operation at a single, or few, wavelengths is required and maximumand/or polarisation insensitive gain is at a premium, for example incascaded amplification where the cumulative effects of polarisationsensitivity of gain are a severe problem. In some applications thewavelength sensitivity of the polarisation insensitivity may be utilisedto advantage. For example in multiply cascaded (with more than threestages) amplification where the signal is polarised, noise will berandomly polarised and at random wavelengths. In a polarisationsensitive system only the noise with the polarisation the same as thesignal will be effectively amplified and the noise on the polarisationwith less gain is effectively lost after several amplification stages.If such a system is rendered polarisation insensitive over a wide passband, then all the noise will be amplified, although of course thesignal benefits also from the polarisation insensitivity. With a deviceaccording to the present invention the device is polarisation sensitiveoff the transmission wavelength and so the reduction in noise gainoperates the same as in a polarisation system, but at the transmissionwavelengths the device has the advantage of polarisation insensitivity,and thus the device will provide a degree of filtering. Also in awavelength division multiplexed system operating on channelscorresponding to the longitudinal mode ripple overlap wavelengths withinthe 3 dB gain peak at which the amplifier is polarisation insensitive,there is a fall off in gain in both TE and TM propagation modes to eachside of the operating wavelengths, and therefore the amplifier willprovide a degree of natural filtering of unwanted wavelengths.

I claim:
 1. A laser amplifier emitting multiple longitudinal modeswithin a 3dB gain envelope, said amplifier comprising a waveguide havinga waveguiding cavity, and an energy source, the waveguiding cavityproviding means for transferring energy from the energy source toamplify the light propagating therein, the waveguiding cavity having aneffective refractive index difference for light propagating in the TEand TM transverse modes such that one of the TE and TM modes has agreater number of longitudinal modes oscillating within said 3dB gainenvelope than the other, and such that at least four longitudinal modesoscillating in the TE mode substantially coincide in wavelength withlongitudinal modes oscillating in the TM mode within said 3dB gainenvelope.
 2. A laser amplifier according to claim 1, wherein thewaveguiding cavity is such that there is an effective optical pathlength difference between the TE and TM modes of at least 2.5×10⁻⁴meters.
 3. A laser amplifier according to claim 1, wherein thewaveguiding cavity comprises an active region which is surrounded byconfinement regions.
 4. A laser amplifier according to claim 3, whereinthe refractive index is varied at the boundary between the active regionand the confinement regions so as to maintain similar confinementfactors for the TE and TM modes.
 5. A laser amplifier according to claim4, wherein the waveguiding cavity includes first and second pairs ofopposite sides and the refractive index variation across the boundary isgreater at said first pair of opposite sides than at said second pair ofopposite sides.
 6. A laser amplifier according to claim 5, wherein theactive region is bounded by a graded index confinement region.
 7. Alaser amplifier according to claim 5, wherein the confinement regions atsaid first pair of opposite sides provide a greater refractive indexstep relative to the active region than is provided by the confinementregions at said second pair of opposite sides.
 8. A laser amplifieraccording to claim 3, wherein the active region comprises birefringentmaterial.
 9. A laser amplifier according to claim 1, wherein thelongitudinal modes for the TE and TM modes are substantially coincidentat least once every ten modes.
 10. A laser amplifier according to claim1, wherein the ratio of the confinement factors of said waveguidingcavity for the TE and TM modes is at least 0.9.
 11. A laser amplifieraccording to claim 5, wherein the active region is asymmetric having anaspect ratio greater than 10:1.
 12. A laser amplifier according to claim11, wherein the aspect ratio of the asymmetric active region is at least20:1.
 13. A wavelength division multiplex system including a laseramplifier according to any one of claims 1, 2, 3, 4, 5, 6, 7, 8, 9, 10,11 or 12.